Directional Coupler Circuit With Flat Response and High Directivity
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Solution Overview
Problem
Traditional directional couplers face limitations in directivity and frequency range, leading to narrow bandwidth and significant coupling variations, which restrict their application in measuring RF power in coaxial transmission lines effectively.
Innovation Solution
A directional coupler circuit design incorporating a capacitive divider, a coil with mutual inductance, and operational amplifiers for both forward and reflected paths, which includes a voltage operational amplifier and transimpedance operational amplifiers to isolate and measure RF power in both directions, while using a shield to isolate the coil from the electric field and employing feedback resistors and capacitors for tuning, resulting in improved frequency range and coupling flatness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional directional coupler designs are used, then the device structure is simple, but the directivity and frequency range are limited
Solution Approach 1:
The directional coupler is divided into separate functional paths: a forward path with a forward transimpedance amplifier and a reflected path with a reflected transimpedance amplifier. Each path independently processes signals in its respective direction, allowing high directivity through dedicated signal separation while maintaining manageable complexity through modular design.
Solution Approach 2:
Transimpedance amplifiers are introduced as intermediary components between the coupling elements and the output ports. These amplifiers convert current signals to voltage signals and provide impedance matching, enabling high directivity through improved signal separation while the use of standard amplifier components keeps the overall device complexity reasonable.
2Adaptability or versatility
If traditional directional coupler designs are used, then the device structure is simple, but the frequency range is narrow
Solution Approach 1:
The directional coupler employs transimpedance amplifiers with frequency-compensated designs that dynamically adjust their performance characteristics across different frequencies. The amplifiers are designed to maintain stable operation over a wide frequency range (150:1 ratio), allowing the device to adapt to various frequency conditions while using standard electronic components that keep complexity manageable.
Solution Approach 2:
The design utilizes operational amplifiers with adjustable gain and bandwidth parameters that can be optimized for different frequency ranges. By changing the feedback resistor and capacitor values in the transimpedance amplifiers, the device can maintain high directivity and flat frequency response across a wide frequency spectrum without requiring complete redesign for each frequency band.
3Manufacturing precision
If traditional directional coupler designs are used, then the device structure is simple, but the coupling varies significantly with frequency
Solution Approach 1:
The transimpedance amplifiers incorporate feedback networks with carefully selected resistors and capacitors that compensate for frequency-dependent variations in coupling. The feedback mechanism actively corrects for frequency-related deviations, maintaining flat coupling response across the frequency range while using standard feedback components that do not significantly increase device complexity.
Solution Approach 2:
The design replaces traditional mechanical or passive LC tuning elements with active electronic transimpedance amplifiers that provide frequency compensation through electronic feedback. This substitution achieves superior coupling flatness across wide frequency ranges while using integrated circuit components that reduce overall device complexity compared to precision mechanical tuning systems.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design achieves high directivity (>30 dB) and minimal coupling variation (<10%) over a wide frequency range (150:1), enabling effective measurement of RF power in both forward and reflected directions without frequency dependence, using a single coil and low-value capacitors, thus expanding the practical frequency range and reducing component costs.
Implementation Method 1
a coil having mutual inductance with the center conductor for sampling current travelling in a forward direction and a reflected direction on the transmission line
Implementation Method 2
a capacitive divider for sampling a voltage on a center conductor of a transmission line
Implementation Method 3
employing feedback resistors and capacitors for tuning, resulting in improved frequency range and coupling flatness
Data Source
AI summary
Disclosed is a directional coupler circuit that has improved performance in the areas of directivity and frequency range, when compared with conventional directional couplers. The disclosed directional coupler circuit improves performance in at least two ways. The first is by the use of the voltage operational amplifier that unloads the capacitive voltage divider. The second is using forward and reflected transimpedance operational amplifier circuits to provide a short circuit to the coupling coil, namely a forward direction current sample circuit having a forward transimpedance operational amplifier and a reflected direction current sample circuit having a reflected transimpedance operational amplifier.